1. Field of the Invention
The present invention generally relates to coating systems suitable for protecting components exposed to high-temperature environments, such as the hostile thermal environment of a gas turbine engine. More particularly, this invention is directed to a low-temperature thermal spraying process for depositing an environmental barrier coating system comprising a mullite-containing bond layer and a barium-strontium-aluminosilicate topcoat.
2. Description of the Related Art
Higher operating temperatures for gas turbine engines are continuously sought in order to Increase their efficiency. Though significant advances in high temperature capabilities have been achieved through formulation of iron, nickel and cobalt-base superalloys, alternative materials have been investigated. For example, composite materials are currently being considered for such high temperature applications as combustor liners, vanes, shrouds, airfoils, and other hot section components of gas turbine engines. Of particular interest are silicon-based composites, such as silicon carbide (SIC) as a matrix and/or reinforcing material.
Thermal barrier coatings (TBC""s) are widely used on hot section components to reduce their operating temperatures and thermal gradients through the component materials. Hot section components formed of Si-containing materials further benefit from a protective coating capable of inhibiting the major mechanism for degradation of Si-containing materials subjected to corrosive water-containing environments, namely, the formation of volatile silicon monoxide (SiO) and silicon hydroxide (Si(OH)4) products. Consequently, besides low thermal conductivity, a critical requirement of a coating system for a Si-containing material is stability in high temperature environments containing water vapors. Other Important properties for the coating material include a coefficient of thermal expansion (CTE) compatible with the Si-containing material, low permeability to oxidants, and chemical compatibility with the Si-containing material and silica scale formed from oxidation. As a result, protective coatings for gas turbine engine components formed of Si-containing materials must provide environmental protection, and a coating system having this function will be referred to below as an environmental barrier coating (EBC) system.
Various single-layer and multilayer EBC systems have been investigated for use on Si-containing substrates. Coatings of zirconia partially or fully stabilized with yttria (YSZ) as a thermal barrier layer exhibit excellent environmental resistance. However, YSZ does not adhere well to Si-containing materials (SiC or silicon) because of a CTE mismatch (about 10 ppm/xc2x0 C. for YSZ as compared to about 4.9 ppm/xc2x0 C. for SiC/SiC composites). Mullite (3Al2O3xc2x72SiO2), barium-strontium-aluminosilicate (BSAS; (Ba1xe2x88x92xSrx)Oxe2x80x94Al2O3xe2x80x94SiO2) and other alkaline earth aluminosilicates have been proposed as protective coatings for Si-containing materials. For example, U.S. Pat. No. 5,496,644 to Lee et al. and U.S. Pat. No. 5,869,146 to McCluskey et al. disclose the use of mullite and U.S. Pat. Nos. 6,254,935, 6,365,288, 6,387,456 and 6,410,148 to Eaton et al. disclose the use of BSAS as outer protective barrier coatings for silicon-containing substrates. In the Eaton et al. patents, BSAS barrier coatings are described as being bonded to a silicon-containing substrate with an intermediate layer (bond layer) that may be, among other possible materials, mullite or a mixture of mullite and BSAS.
Lee et al. teach that a plasma-sprayed mullite-containing coating must be deposited on a substrate heated to at least 800xc2x0 C., more preferably at least 1000xc2x0 C., in order to ensure that mullite is deposited in its crystalline form. According to Lee et al., shrinkage of mullite during the crystallization of amorphous mullite is the key factor causing cracking and debonding of a mullite coating during thermal cycling. Mullite is said to immediately crystallize as it solidifies on a substrate maintained at the temperatures prescribed by Lee et al., thereby avoiding the transition from amorphous to crystalline phases. McCluskey et al. teach that, by selecting appropriate powder size and spray parameters, the applied mullite coating containing at least 85% by volume of crystalline phase is crack-free if the substrate is heated to a temperature of about 845xc2x0 C. to about 935xc2x0 C. In each of the patents to Eaton et al., emphasis is also placed on the importance of thermal spraying mullite and BSAS-containing coatings on a substrate maintained at a sufficiently high temperature to ensure that BSAS is deposited in its monoclinic celsian crystalline phase, which has a CTE closer to SiC/SiC CMC than the amorphous BSAS phase. In U.S. Pat. Nos. 6,254,935 and 6,365,288 to Eaton et al., a substrate temperature of at least 1100xc2x0 C. is said to be required for the deposition of a mullite-BSAS bonding layer and a BSAS barrier layer. When depositing the BSAS layer, these patents teach that the substrate must be held at at least 1100xc2x0 C. for a period of at least 15 minutes after deposition to develop greater crystallinity in the BSAS layer and substantially eliminate the formation of cracks. In U.S. Pat. Nos. 6,387,456 and 6,410,148 to Eaton et al. a substrate temperature of at least 870xc2x0 C. is disclosed as being required when depositing a mullite-BSAS bonding layer and a BSAS barrier layer. In view of the teachings of Lee et al., mullite would be expected to deposit in its crystalline form when thermal sprayed under the temperature conditions taught by the patents to Eaton et al.
A drawback to the teachings of Lee et al., McCluskey et al. and Eaton et al. is that large components are difficult to maintain at such high temperatures during a thermal spray process. Nonetheless, in view of the prevailing wisdom as taught by Lee et al., McCluskey et al. and Eaton et al., mullite and BSAS-containing coatings have been thermal sprayed on substrates maintained at 800xc2x0 C. or more in order to produce EBC systems with desirable mechanical integrity.
The present invention provides a process for depositing a coating system suitable for use as an EBC on various substrate materials, particularly those containing silicon and intended for high temperature applications such as the hostile thermal environment of a gas turbine engine. Examples of such substrate materials include those with a dispersion of silicon carbide, silicon nitride and/or silicon reinforcement material in a metallic or nonmetallic matrix, as well as those having a silicon carbide, silicon nitride and/or silicon-containing matrix, and particularly composite materials that employ silicon carbide, silicon nitride and/or silicon as both the reinforcement and matrix materials, e.g., SiC/SiC ceramic matrix composites (CMC).
The invention is generally directed to the deposition of mullite-containing and alkaline earth aluminosilicate-containing coatings. For example, the invention is applicable, though not limited, to depositing coating systems of the type disclosed in U.S. Pat. Nos. 6,254,935, 6,365,288, 6,387,456 and 6,410,148 to Eaton et al., and which therefore comprise an outer coating of BSAS and a bond layer of mullite or a mixture of mullite and an alkaline earth aluminosilicate, e.g., BSAS. The process of this invention comprises depositing the bond layer and outer coating by thermal spraying while maintaining the substrate at a temperature of less than 800xc2x0 C., and preferably less than 500xc2x0 C., by which an EBC system is produced having desirable mechanical integrity and exhibiting resistance to cracking during thermal cycling.
EBC coatings of this invention can be deposited by thermal spray techniques such as air plasma spray (APS) and low-pressure plasma spray (LPPS), the latter of which is also known as vacuum plasma spraying (VPS). As evidenced by the teachings of Lee et al., McCluskey et al. and Eaton et al., substrate temperatures during thermal spray deposition are known to have a significant effect on the microstructure and mechanical integrity of a thermal sprayed coating. Generally, substrate temperatures affect the amount of thermal stress that occurs as the molten particles impact and solidify onto the substrate during coating deposition, and the amount of cooling stress generated from any thermal expansion mismatch between the coating and substrate materials during subsequent cooling to room temperature. These stresses are mostly tensile in nature and are the cause of through-thickness cracks in the resulting coating if these stresses exceed the tensile strength of the coating materials. Substrate temperature also has an impact on the cooling rate of the coating material after the molten particles are deposited on the substrate, which can result in different crystallinity or phases in the coating. For some coating materials, the thermal expansion coefficient of the crystalline phase is very close to that of the substrate, resulting in lower residual stresses if the coating can be maintained in the crystalline phase after deposition and during subsequent cooling to room temperature.
The above considerations apply to thermal spraying of materials used in EBC coatings, and have lead to the conclusions espoused by Lee et al., McCluskey et al. and Eaton et al. that substrate temperatures in excess of 800xc2x0 C. are required to reduce the resulting stresses, and by Lee et al. and Eaton et al. that high substrate temperatures are needed to increase the crystallinity of mullite-containing and BSAS-containing coatings, with the benefit being a decrease in the occurrence of cracks upon cooling and during thermal cycling of a coating system containing one or both of these coating materials. In the present invention, particle temperature during deposition is identified as another key parameter that affects the resulting stresses and the crystallinity or phases of these coating materials. In particular, it is theorized that in-flight particle temperature will affect coating crystallinity, and particle temperatures as the particles impact the substrate will affect the resulting stresses. In the present invention, it is believed that the crack-free coatings can be obtained at lower substrate temperatures if adequate steps are taken to reduce the in-flight as well as impact particle temperatures during deposition in order to reduce the resulting stresses and maintain the coating crystallinity, especially in the case of a mullite-containing bond layer. These steps include the use of lower gun power, longer spray distance and larger particle size.
With the present invention, thermal spraying of an EBC system on substrates maintained at temperatures of about 350xc2x0 C. and less has been proven to be feasible and capable of producing a substantially crack-free coating. The invention is particularly suited to depositing a composite mullite bond layer containing an alkaline earth aluminosilicate, a notable example of which is a mullite-BSAS coating containing, in weight percent, about 80% mullite and about 20% BSAS, on which an alkaline earth aluminosilicate topcoat, preferably BSAS, is then deposited. As a result, the present invention provides a process by which an EBC comprising a mullite-containing layer and BSAS topcoat can be deposited at substrate temperatures far lower than previously thought possible. EBC""s containing a mullite-containing bond layer and BSAS topcoat that are deposited at substrate temperatures in accordance with this invention have been shown to exhibit mechanical integrity and microstructures comparable to EBC""s deposited at substrate temperatures prescribed by the prior art, e.g., above 800xc2x0 C.
Other objects and advantages of this invention will be better appreciated from the following detailed description.